Some physical link specifications incorporate self-test hooks that can be used to collect, for example, eye diagrams, equivalent time oscilloscope traces and/or bit error rate diagrams. In order to use these hooks, the physical link under test must complete a training sequence. However, if a link is non-functional the training sequence cannot be completed and the self-test hooks cannot be used.
One example of a self-test hook is the compliance state as documented in the PCI Express standards. The PCI Express standards refer to documents available from the PCI Special Interest Group of Portland, Oreg. The compliance state of the PCI Express standards provides a mechanism by which an oscilloscope probe or other instrument can be connected to a physical link. As the link detects the probe or other instrument it may enter the compliance state. In the compliance state the transmitter sends a predetermined pattern on each of the lines on which the probe or other instrument has been detected. In the compliance state external test equipment can monitor the functionality of the transmitter.